The role of electrical switching devices, such as circuit-breakers, is to decouple a number of consumers from a voltage supply network when a particular fault occurs. The classic fault is the occurrence of a short-circuit current, and circuit-breakers are conventionally designed to move a contact element in the event of such a short-circuit current and thus decouple the connection between consumers and voltage supply network.
However, such a decoupling of consumers and voltage supply network is further also desirable in other situations. In particular, it may be that in the event of an undervoltage in the voltage supply network, the voltage supply network is disconnected from the consumers. In order to provide a functionality for this, shunt releases are used. This means that circuit-breakers, in particular compact circuit-breakers, can be switched off with the aid of shunt releases. These are preferably electromagnetic releases, which usually have an energy store and which trip either in the presence of a voltage (open-circuit shunt releases) or when a critical voltage is undershot (undervoltage releases). Open-circuit shunt releases also include direct-acting electromagnetic releases which work without an additional energy store.
Such shunt releases are typically designed as a module which can be attached to a recess, for example a bay, of the circuit-breaker. Part of the release mechanism in a circuit-breaker is traditionally a latching mechanism. Shunt releases act precisely on this latching mechanism or on a contact element of the latching mechanism and thus effect a decoupling of the consumer from the voltage supply network by the circuit-breaker. To this end they are fitted with a mechanical release element, e.g. a plunger. The release element, namely in particular the plunger, is retracted in a non-released state. For release the release element travels out of a housing of the shunt release.
Circuit-breakers or compact circuit-breakers vary considerably in size. They differ essentially in their current-carrying capacities and thus significantly in their size. Large compact circuit-breakers require larger energy stores in the shunt releases, to be able to conduct and switch high currents. In order to release a large compact circuit-breaker, for example an MCCB (MCCB=Molded Case Circuit Breaker) by means of a shunt release, a higher release energy is thus also required.
In consequence different sizes of shunt release also exist for different sizes of circuit-breaker. Although these do not need to be assigned explicitly to each size of circuit-breaker, nonetheless no supplier of circuit-breakers currently uses exactly the same shunt release for every size of circuit-breaker. The reasons for this are as follows:                Differing requirements for forces and paths for releasing a circuit-breaker,        Small circuit-breakers require less release energy and a restricted installation space, whereas large circuit-breakers need a higher release energy and more installation space,        Different interfaces for release and for reset.        